This invention generally relates to printing antialiased images and other image structures having gray edges and, more particularly, to an edge restoration method that compacts gray pixels into neighboring edges.
Antialiasing in the context of digitizing line art and certain graphical image structures is well known as a method of using intermediate levels of intensity to achieve subpixel positioning to estimate the edges of lines and text. A major benefit of antialiased images is that high contrast, saturated objects are surrounded with pixels possessing intermediate gray values that visually suggest a higher resolution of object edges.
Generally, antialiased images contain gray edges that are a single pixel wide. Gray edges that are a single pixel wide can be well rendered using anti-aliasing rendering (AAR) techniques. However, some image processing functions can introduce gray pixels to an antialiased edge thereby broadening the edge to two or more pixels in width. For example, electronic image registration correction is an image processing technology used to compensate for misregistration of images written onto a photoreceptor. The misregistration could be due to a variety of sources, such as bow differences between raster output scanners in a multiple output scanner imaging system. Electronic registration correction often uses interpolation of pixel values to compensate for pixel positioning errors. The interpolation can have the effect of slightly blurring an edge, thereby increasing its gray content. If an input edge is antialiased, then the interpolated image may have gray edges that are two pixels wide. Additionally, scanning processes can blur an edge such that the edge contains two gray pixels. Similarly, some super-resolution processing techniques can blur an edge to two gray pixels in width.
Current antialias rendering techniques do not recognize gray edges that are two or more pixels wide, and thus the blurred edges will be halftoned. Halftoning antialiased edges that are two pixels wide can yield an undesirable beaded structure. Thus, what is needed is a method to compact a gray edge that is two pixels wide to a state where it is one pixel wide so that it may be well rendered with AAR.
In accordance with the present invention, there is provided a method for compacting diffuse gray edges. The method identifies a group of interested pixels with an observation window within continuous tone image data. The method then operates on the pixels within the observation window by: compacting horizontal features within the observation window; compacting vertical features within the observation window; and compacting corner features within the observation window.
In accordance with a second aspect of the invention there is provided, in a printing system having a digital front end for processing image data to generate print ready data, a method for compacting a diffuse gray edge. The method includes the steps of: receiving image data; identifying a target pixel within said received image data; analyzing pixels neighboring the target pixel to determine if the target pixel is within a diffuse gray edge; and compacting gray pixels within the diffuse gray edge.